Studies are performed of green fluorescent (GF) and other photoactive proteins, with emphasis on elucidating the photochemical cycles and mechanisms of proton transfer thought to underlie the complex spectroscopic and related behaviors observed in these important biochemical markers. The rapidly growing body of literature in this area is studied, and selected fast-fluorescence and related measurements in both single-molecule and bulk phases interpreted in a research program employing theoretical and computational methods. Attention is addressed to differing behaviors of mutant variants of GFP, of photoactive yellow proteins, and of recently developed "non-natural" amino acid fluorescent probes of internal solvation at specific protein sites. The recognized importance of GFP as a cloneable 'non-invasive marker, which can be employed in living cells as a reporter of gene expression and protein localization, in the detection of protein-protein interactions, and as an intra-cellular sensor of various physiological conditions, suggests its study can provide a broadly-based introduction to the properties of proteins more generally. Better understanding of the detailed workings of GFP may contribute to development of new improved fluorescent markers for specified purposes, including detection of metastases from single-cell to whole-body scanning levels, among other potential diagnostic benefits.